Electron multiplier gain control



J. A. BUCKBEE ELECTRON MULTIPLIER GAIN CONTROL Filed DeC. 2, 1943 Dec. 16, 1947.

ATTORNEY Patented Dec. 16, 1947 ELECTRON MULTIPLIER GAIN CONTROL John A. Buckbee, Fort Wayne, Ind., assignor, by mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application December 2, 1943, Serial No. 512,555

Claims. l

This invention relates tosignal ampliiiers and particularly to the automatic gain control of an electron multiplier type of amplier.

It is customary to employ multistage static electron multipliers to' amplify many diiferent typesof signals. The manner in which the electron` multipliers are connected and operated depende upon the particular use to which they are put. Regardless of the type of signal to be amplied or the faithfulness with which the signal amplification is to be effected, a multistage multiplier is essentially a device which is operated by impressing positive potentials of increasing values to successive ones of the secondary emissive multiplier electrodes. In many cases it is desirable to maintain a substantially uniform potential difference between successive multiplier electrodes. This type'of operation may be achieved in one way by the use of independent sources oi potential for each pair of electrodes. In this manner the successivedifferences of potential between any two electrodes is not materially affected by the electron current 'ow between these two electrodes or by the current conduction in any other part of the multiplier. Thus, the overall multiplication ratio of the device remains substantially constant throughout a considerable range of variation of the electron' concentration at' the multiplier input. The voltages developed in the output circuit of the multiplier' then are substantially directly proportional to the primary electron concentrations at the multiplier input. Such a device is said to have a linear operating characteristic.

There are, however, some uses for an electron multiplier where linearity between input and output is unnecessary or even undesirable. One such application of an electron multiplier'is presented frequently 'm a television system where the multipli'er is to be used in conjunction with a camera tube of theA dissector type. When television apparatus. of' such a character is to be used in astudio or similar location it isa comparatively simple matter to control the subject illumination in a manner suitable to effectv the generation of video signal voltages representing light and shade values of the subject having any desired contrast, However, where the apparatus is to be used in mobile installations the subject frequently is provided with natural illumination, the inten- (Cl. Z50-27) sity of which is not susceptible of control. The

subject illumination level or intensity may change many times during the operation ofsuch mobile equipment because of' cloud movements, haze conditions andthe like. If, in such a situ,-

ation the entire television system, including the reproducing apparatus, is adjusted for relatively weak light conditions and there is a sudden increase in the subject illumination intensity, it is likely that some part of the system may become so greatly overloaded as to deleteriously affect the reproduced picture. In such installations it then becomes necessary to monitor the television signals and to eilect the desired control of the signal voltages to provide a suitable light level in the image reproduced under the control of such signals.

If a multiplier having a linear operating characteristic is employedl to amplify the generated video signals, the peak-to-peak value of these signals will vary as a function of the variation in the intensity of the subject illumination. The expression peak-to-peak value of the video signals denotes the difference between the voltage representing picture black and the voltage representing picture White. In such a system it is necessary to employ apparatus either manually or automatically operable to convert the video signals having variable peak-to-peak values to signais having a substantially constant peak-to-peak value in order to enect the desired contrast control,

It is an object of the invention, therefore, to provide an automatic gain control for a multistage electron multiplier whereby to eiiect the production of a signal voltage having a peak-topeak value not exceeding a predetermined value, irrespective of a variation of the diiference between the lowest and highest primary electron concentrations.

In accordance with the invention, there is provided an electron multiplier having a secondary electron emissive electrode upon which there is impressed a unidirectional accelerating voltage. In order to automatically control the gain of the multiplier there is provided means for impressing upon the electrode an additional unidirectional' voltage which is controlled by the secondary electron current ilow in a manner to vary the eectiveness of the electrode as a function of the magnitude of the electron current ilow in the multiplier.

In accordance with the illustrated embodiment.

of the invention, a multistage electron multiplier is provided with electrode potentials derived from.

a voltage divider connected between the terminals oi'a source of unidirectional voltage. Each of a plurality of the connections between the voltage divider and the respective multiplier electrodes includes. an impedance device through which is caused to ow the electron current between the associated multiplier electrodes. In this manner the potentials applied to such electrodes are varied substantially instantaneously under the control of the interelectrode electron current flow so that the peak-to-peak value of the video signals developed in the output circuit of the multiplier also is varied substantiallyinstantaneously in a manner to accomplish the type of gain variation known as gamma control. Such operation is known to those skilled in the art as one by which the relative contrasts between picture elements are varied according to the illumination intensity of the elements. In other words, the developed signals are subjected to a contrast control which varies as a function of the intensity of subject illumination.

In accordance with an additional feature of the invention the impedance devices included in the connections between the voltage divider and certain of the multiplier electrodes may be bypassed by connecting suitable condensers between the impedance devices and ground. By suitably adjusting the values of the impedance devices and the by-pass condensers, the time constants of the circuits may be arranged in such a manner to effect a relatively slow rate of change of the effective accelerating potentials impressed upon the respective multiplier electrodes. Such an arrangement produces a similar relatively slow rate of change of the multiplier gain whereby to accomplish the type of average gain control of the multiplier commonly known as AVC.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying' drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing;

Fig. 1 is a diagrammatic representation oi a dissector tube embodiment of the invention;

Fig. 2 is a schematic wiring diagram showing the electrical characteristics of the pertinent components of a dissector tube drawn to an enlarged scale; and

Fig. 3 is a graphical representation of the performance ofr a combined dissector tube-multiplier embodiment of the invention.

Having reference now to Fig. 1 of the drawing, there is illustrated a sectional view of a dissector tube provided with an evacuated envelope II. Mounted within and adjacent to one end of the envelope is a photoelectric cathode I2. Also, within the envelope and adjacent to the end opposite the cathode there is mounted a multistage electron multiplier housed within a metallic shielding anode I3. The anode is provided with a recessed portion I4 at the end of which is formed a primary scanning aperture I5 which is disposed substantially centrally with respect to the cathode I2 and faces the cathode. An accelerating anode is provided in the form of an interior wall coating I6 between the cathode I 2 and the anode I3.

A focusing coil I'I is disposed on the outside of the tube envelope II in a manner to surround substantially completely the space within the tube between the cathode and the anode. A battery I8"'or other suitable source of electrical energy is connected to the focusing coil for the energization thereof to establish the desired focusing field within the tube envelope whereby an electron image is formed substantially in the plane of the anode I3. The tube als() is prQVlClQd with horizontal and vertical scanning coils I9 and 2I, respectively. Each of these coils is energized by respective sources 22 and 23 of appropriate saw-toothed wave form voltages.

In conjunction with the dissector tube there also is provided an optical system represented by a lens 24, whereby an optical image of a subject 25 is focused into the plane of the photoelectric cathode I2.

Referring now to Fig. 2 of the drawing, a more detailed description of the electrical connections to the photoelectric cathode I2 and the electron multiplier will be given. The multiplier which is housed within the anode shield I3 comprises a plurality of box-like electrodes such as 26, 21, 28 and 29. In the case of a so-called elevenstage multiplier the device includes ten of such box-like electrodes, the eleventh stage ordinarily being in the form of a plate such as 3|. A collecting electrode of suitable form such as a grid 32 is located between the tenth stage electrode 29 and the eleventh stage electrode 3I. The first stage electrode 26 is provided with a secondary scanning aperture 33 in alignment with the primary scanning aperture I5 formed in the anode shield I3. The secondary aperture ordinarily is smaller than the primary aperture, being of substantially the same size as the desired elemental scanning area.

Suitable accelerating potentials are impressed upon the multiplier electrodes and the photoelectric cathode by appropriate connections to taps on avoltage divider which, as illustrated, comprises the series connection of resistors 34 to 45, inclusive. A source of relatively high unidirectional voltage such as a battery 46 is connected to the terminals of the voltage divider. The negative terminal of the battery 46 is connected to the photoelectric cathode I2. The junction point between resistors 34 and 35 is connected to the shielding anode I3 and also, by means of an internal connection such as represented by a conductor 47, to the first stage multiplier electrode 26. Thus, the anode shield and the first stage electrode are operated at the same positive potential with respect to the cathode I2. Similarly, the other multiplier `electrodes are connected, respectively, to other taps on the voltage divider in a manner to impress increasingly higher positive potentials on succeeding multiplier electrodes.

Some of these connections include series irnpedance devices in accordance with the instant invention. For example, the fifth stage electrode 21 is connected through an ampedance device such as a resistor 48 to the junction point between voltage divider resistors 38 and 39. Likewise, the seventh stage electrode 28 is connected through a. resistor 49 to the junction point between voltage divider resistors 40 and 4I, The connection of the sixth stage or intervening electrode is made directly to the junction point between voltage divider resistors 39 and 4I) and does not include an impedance device such as a resistor. The series resistors 4'8 and 49 may be by-passed to ground by means of condensers 5I and 52, respectively. These condensers together with their associated series resistors change the time constant of gain control voltage in a manner and for a purpose to be described.

The eleventh stage electrode 3l is connected to the junction point between voltage divider resistors 44 and 45. The other terminal of the resistor 45 is connected to the grounded positive terminal of the battery 46 and also to one terminal of an output resistor 53. The other terminal of the output resistor is connected to the collecting electrode 3'2 of the multiplier and also to one of a pair of output circuit terminals 54 of which the other terminal is connected to ground.

Referring now to the operation oi the illustrative embodiment of the invention, the elec-` tron streams from the elemental areas of the photoelectric cathode i2 are directed successively through apertures l and 33 under the control of the scanning coils lil and 2l in accord ance with well known practice. The impact oi these primary electrons upon the secondary electron emissive surface of the rst stage multiplier electrode 26 produces multiplied members of secondary electrons. These secondary electrons are drawn successively to the other eiectrodes of the multiplier under the control of the accelerat-v ing voltages applied between the electrodes. As a result of the impact of these electrons upon successive electrodes an electron multiplication is eiTected.

So long as each of the accelerating electrode potentials remains at a substantially constant value suilcient to remove all of the electrons from the originating electrode, the multiplication ratio of the originating multiplier stage and of the entire multiplier apparatus does not change appreciably with variations of the primary electron concentrations admitted to the multiplier. In such a case the voltages developed in the output resistor 53 correspond substantially linearly to the primary electron concentrations.

According to the present invention, however, the series resistors d8 and i9 are traversed by electron currents whereby to develop additional unidirectional voltages for impression upon the fth and seventh multiplier stages 2l and 23, respectively. These additional voltages vary in magnitude in accordance with the electron current flow. The manner in which these facilities automatically control the CTain of the multiplier may be more fully understood by reference to Fig.

The solid line curve 55 represents the voltages impressed upon the various multiplier electrodes in a case where the concentrations of the primary electrons admitted to the multiplier are of sufficient magnitude to render operable the auto matic gain control facilities provided in accordance with this invention. It is seen that between each pair of the successive electrodes l to l and also between each pair of the successive electrodes 3 to l l the impressed accelerating voltages are of substantially equal values. However.4 with respect to electrodes 4 to 8, the slope of the curve 55 Varies, indicating unequal values of potentials impressed upon these electrodes. For relatively low primary electron concentrations the. voltages applied to the electrodes 'l to 8 also are substane tially equal to the potentials impressed upon the other electrodes as indicated by the broken line portion of the, curve.

Considering the fth stage multiplier electrode 2l, the secondaryA emissive surface thereof is impacted by electrons originating inl the preceding multiplier stage. A multiplied number of secondary electrons thus are produced and are ree moved from the electrodeV 2l under the control. of the effective accelerating voltage impressed between this and the succeeding electrode.l As a result, there are more. electrons leaving the electrode 21 than there are arriving. The difference in these numbers of electrons is derived from the battery 46 and isv supplied through the associated circuits, including the Voltage divider.

(ill

ditional electrons supplied to the electrode 2 also ilow through the series resistor 48, thereby producing a current flow through this resistor in a direction to increase the positive potential of the fifth stage multiplier electrode. Ordinarily, without the inclusion of the series resistor 48, the nominal or primary potential impressed upon the fifth stage electrode 2? is represented by the point 56 on the voltage curve of Fig. 3. It is seen that this voltage is substantially midway between the preceding and succeeding electrode voltages- However, the increase in the potential impressed upon the electrode 21 under the control of the electron current in the resistor d8 produces a voltage indicated by the point 5l. Consequently, the accelerating potential available to remove electrons from the electrode 27, instead of hav ing the nominal value A, is reduced by the value B which is the increase in the voltage impressed upon this electrode. Hence, the effective accelerating potential is indicated by the value A. The accelerating potential A is of insuflicient magnitude to remove all of the secondary electrons emitted by the electrode 2?. As a result, there is produced a space charge in the vicinity of this electrode which then will exhibit an apparent multiplication ratio which is somewhat lower than the theoretically realizable ratio. In other Words, the impact of a given number of electrons upon the electrode 2i is capable of always producing a certain number of secondary electrons, but the space charge eiect allows only a clecreased number of the secondary electrons to reach the succeeding' electrode for further multiplication,

The fact that the increased accelerating potential impressed upon the electrode 2l increases the velocity of the impacting electrons and thereby increases the number of secondary electrons emitted by this electrode is of little practical importance in the operation of the automatic gain control facilities. The reason for this is that most of the increased number of secondary electrons become part of the space charge which is produced in the manner described.

Fig. 3 also illustrates a similar performance of the multiplier with respect to the seventh stage electrode 28. At relatively low primary electron concentrations the nominal value of the accelerating potential available for withdrawing secondary electrons from this electrode is represented at C. At relatively high primary electron concentrations the potential of the electrode is increased by the amount D, whereby to produce an eiiective accelerating potential C.

Thus, it is seen that so long as the primary electron concentrations do not exceed a predetermined magnitude the effective accelerating potentials for the multiplier electrodes are all of substantially equal values. However, when the predetermined magnitude of electron concentra tions is exceeded, as for example when the average intensity of the subject illumination is n crea-sed, there are produced the described space charge eiects in the vicinity of the multiplier electrodes upon which the voltage divider potentials are impressed by connections including series impedance devices. .As a consequence, the multiplication ratio of such electrodes are varied as inverse functions of the magnitudes of the electron concentrations. It. is apparent, therefore, that a corresponding variation in the overall multiplicaticn ratio of the multiplier also will beI eiected.

Where only the series resistors 48 and 49 are included in the connections between the voltage divider and the multiplier electrodes 21 and 28 the multiplication ratio or the multiplier is Varied in the manner described at a relatively rapid rate. The resulting video signal voltages therefore are compensated for changing illumination levels of the television subject so that there is produced a gamma control. However, if it is desired to produce an AVC to control the multiplier gain in accordance with the average variation in the electron stream concentrations, the by-pass condensers l and 52 may be connected in a manner such as that illustrated and described.

In such a case the rate of change of the multiplication ratio with changing illumination intensities is relatively7 slow and depends upon the time constant of that portion oi the circuit which includes a series resistor such as i8 and a by-pass condenser such as 5|.

It is obvious that the result produced by the described apparatus in accordance with the instant invention may be obtained by including a series resistor in the connection between the voltage divider and a single one of the multiplier electrodes. On the other hand, it is considered to be within the scope of the invention to provide series resistors in the connections between the voltage divider and as many as all of the multiplier electrodes. Also, the by-pass condensers such as 5l and 52 may or may not be used with the respective series resistors in order to obtain the particular type of automatic gain control desired.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the al't that various changes and modications may be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modincations as fall within the true spirit and scope of the invention.

What is claimed is:

1. An automatic gain control for an electron multiplier comprising, a plurality of secondary electron emissive electrodes in said multiplier, means for impressing unidirectional electron accelerating voltages upon each of said respective electrodes, and means for impressing upon some but not all of said electrodes additional unidirectional voltages, each of said additional voltages varying in magnitude in accordance with the electron current flow between respective pairs of said electrodes.

2. An automatic gain control for an electron multiplier comprising, a plurality of secondary electron emissive electrodes in said multiplier, a plurality of primary sources of unidirectional voltage, an additional source of unidirectional voltage varying in magnitude in accordance with the electron current flow between a pair of said electrodes, and connections between said primary voltage sources and each of respective ones of said electrodes, the connection to one of said electrodes including said additional voltage source and the connection to a following one of said electrodes being made directly to one of said primary voltage sources.

3. An automatic gain control for an electron multiplier comprising, a plurality of secondary electron emissive electrodes in said multiplier, a plurality of primary sources oi unidirectional voltage, a plurality of additional sources of unidirectional voltage, each of said additional voltages varying in magnitude under the control of the electron current ilow between respective pairs of said electrodes, and connections between said primary Voltage sources and each of respective ones of said electrodes, some but not all of the connections to a plurality of said electrodes including one of said additional voltage sources and the connection to an intervening one of said electrodes being made directly to one of said primary voltage sources.

4. An automatic gain control for an electron multiplier comprising, a plurality of secondary electron emissive electrodes in said multiplier, a plurality of primary sources of unidirectional voltage, two additional sources of unidirectional voltage, each of said additional voltages varying in magnitude under the control of the electron current iiow between respective pairs of said electrodes, and connections between said primary voltage sources and each of respective ones of said electrodes, each of the connections to two of said electrodes including one of said additional voltage sources and the connection to an intervening one of said electrodes being made directly to one oi said primary voltage sources.

5. An automatic gain control for an electron multiplier comprising, a plurality of secondary electron emissive electrodes in said multiplier, a source of unidirectional voltage, a voltage divider comprising a plurality of resistors connected in series across said source of voltage, and connections between said voltage divider resistors and each of respective ones of said electrodes, the connection to one of said electrodes including a series impedance device and the connection to a following one of said electrodes having substantially no impedance.

6. An automatic gain control for an electron multiplier comprising, a plurality of secondary electron emissive electrodes in said multiplier, a source of unidirectional voltage, a voltage divider comprising a plurality of resistors connected in series across said source of voltage, connections between said voltage divider resistors and respective ones of said electrodes, each of the connections to a plurality of said electrodes including a series impedance device and the connection to an intervening one of said electrodes being substantially without impedance, and a by-pass condenser connected to one of said last-named series resistors.

'7. An automatic gain control for an electron multiplier comprising, a plurality of secondary electron emissive electrodes in said multiplier', a source of unidirectional voltage, a voltage divider comprising a plurality or resistors connected in series across said source of voltage, connections between said voltage divider resistors and respective ones of said electrodes, each of the connections to two of said electrodes including a series resistor and the connection to an intervening one of said electrodes being substantially non-resistive, and a by-pass condenser connected to one of said last-named series resistors.

8. An automatic gain control for an electron multiplier comprising, a plurality of secondary electron emissive electrodes in said multiplier, a source of unidirectional voltage, a voltage divider comprising a plurality of resistors connected in series across said source of voltage, connections between said voltage divider resistors and respective ones of said electrodes, each of the connections to a plurality oi said electrodes' including a series resistor and the connection to an intervening one of said electrodes being substans tially non-resistive, and a by-pass condenser connected to each of said last-named series resistors.

9. An automatic gain control for an electron multiplier comprising, a plurality of secondary electron emissive electrodes in said multiplier, a source of unidirectional voltage, a voltage divider comprising a plurality of resistors connected in series across said source of Voltage, connections between said voltage divider resistors and respective ones of said electrodes, the connection to one of said electrodes including a series resistor and the connection to a following one of said electrodes being substantially non-resistive, and a by-pass condenser connected to said last-named series resistor.

10. An automatic gain control for an electron multiplier comprising, a plurality of secondary electron emissive electrodes in said multiplier, asource of unidirectional voltage, a voltage divider comprising a plurality of resistors connected in series across said source of voltage, connections between said voltage divider r'esistors and respective ones of said electrodes, each of the connections to two of said electrodes including a series resistor and the connection to an intervening one of said electrodes being substantially nonresistive, and a icy-pass condenser connected to each of said last-named series resistors.

JOHN A. BUCKBEE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

